disk_setup could deal with non-deterministic device naming on EC2 nitro instances

[Nils Meyer]

Cloud Provider: Amazon Web Services

As is documented in [1], instances on the nitro type hypervisor don't attach the NVME disks in deterministic order, yielding a different order of disks, example [2]. This makes it somewhat difficult to format and partition volumes since you don't know the volume ids beforehand when creating an instance (in an Autoscaling group for example).

My current thinking is that maybe a sort of special device name (much like swap / ephemeralX) coul be used to locate a device, for example ebs:root for the root drive - which is easy to detect, ebs:size=12G[0] for the first volume found with 12GiB size. With an appropriate instance profile and boto3 more elaborate selectors are conceivable (for example based on tags).

Further complicating things is that the metadata endpoint doesn't expose the correct device names, opting instead for fantasy names (sda1 for the root volume, sdX for other volumes).

My Workaround for a 2 volume instance: Try and format both devices, then mount the disk by label:
#cloud-config
fs_setup:
  - label: mylabel
    device: /dev/nvme0n1
    filesystem: xfs
    partition: none
    overwrite: false
  - label: mylabel
    device: /dev/nvme1n1
    filesystem: xfs
    partition: none
    overwrite: false

mounts:
- ["/dev/disk/by-label/mylabel", "/mnt/label", "xfs", "defaults"]

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/nvme-ebs-volumes.html
[2] https://gist.github.com/nilsmeyer/eddcfa4b7fc5b04ebc0be9eaa3c7b7dd

[Ryan Harper (raharper)]

Thanks for filing the issue and providing your workaround.

[Jan Brouwers (janbrouwers)]

running into the same issue.
The workaround does not work for me as I need 2 drives formatted with different filesystems.

Worse, there doesn't seem to be a way to identify the drives from the ec2 metadata.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3380